It is well known that curable polyester resin systems are widely utilized in making fiber reinforced thermoset articles. The polyesters employed in these resin systems are generally the reaction products of a dicarboxylic acid or anhydride with a polyhydric alcohol. Fiber reinforced molding composites based on polyester resins systems have been advantageously employed in both bulk molding compound (BMC) formulations and sheet molding compound (SMC) formulations. A technical improvement that has made a significant contribution to commercial polyester molding technology is the use of low profile additives to reduce shrinkage during the curing reaction and to thereby improve the dimensional stability and surface smoothness of the fiber reinforced thermoset composite.
A further technical improvement that has made a significant contribution to commercial polyester molding technology is the development of chemically thickened polyester resin systems. Chemical thickening is generally always employed in sheet molding compounds (SMC) and is increasingly being used in bulk molding compounds (BMC). In such systems, a metal oxide or hydroxide such as magnesium oxide or magnesium hydroxide is added to, for example, the uncured polyester/low profile additive along with fillers, glass fiber and other standard materials. The metal oxide or hydroxide interacts with residual acidity in the polyester to build viscosity. The thickened system is relatively tack free and easy to handle, and the high viscosity carries the glass fiber reinforcement to the extremities of the mold during crosslinking of the system.
Other thermoset resin systems based on poly(acrylates) have been developed which can be used to prepare fiber reinforced molding composites by rapid injection molding technology. The poly(acrylates) employed in these resin systems are generally prepared by the reaction of acrylic acid or methacrylic acid or their simple esters with a polyhydric alcohol. Poly(acrylate) resin systems generally have a relatively low viscosity so that they can be used to produce thermoset resin articles containing up to about 75 weight percent of reinforcing fibers by a rapid injection molding process. Chemical thickening is not employed in such poly(acrylate) resin systems.
U.S. Pat. No. 4,327,013 describes curable molding compositions which contain a mixture of (a) one or more fibers with a melting point or a glass transition temperature above about 130.degree. C., (b) a poly(acrylate) characterized by the following empirical formula: ##STR1## wherein R is the hydroxy-free residue of an organic polyhydric alcohol which contains alcoholic hydroxyl groups bonded to different carbon atoms, R.sub.1 and R.sub.2 are independently hydrogen or methyl, and n is 1 to 3, (c) an ethylenically unsaturated monomer which is soluble in and copolymerizable with (b), and (d) an elevated temperature free radical curing catalyst capable of affecting the co-reaction of (b) with (c). This patent additionally describes an injection molding process for producing high performance reinforced thermoset molded articles by (i) forming a mixture of the above-described curable molding composition, (ii) providing the mixture into a heatable mold and (iii) molding the mixture at a temperature sufficient to cause the initiation of the co-reaction of (b) with (c) at a temperature not greater than 150.degree. C. and (iv) completing the molding in less than 6 minutes from the time of the initiation of the co-reaction.
Copending U.S. patent application Ser. No. 279,448, filed July 1, 1981, now abandoned, describes a curable molding composition containing a mixture of (a) a poly(acrylate) characterized by the following empirical formula: ##STR2## wherein R is the hydroxy-free residue of an organic polyhydric alcohol which contains alcoholic hydroxyl groups bonded to different carbon atoms, R.sub.1 and R.sub.2 are independently hydrogen or methyl, and n is 1 to 3, (b) acrylic or methacrylic acid or a functionalized derivative thereof having a molecular weight of less than 300 which is different from (a), and (c) an ethylenically unsaturated monomer which is soluble in and copolymerizable with (a) and (b) and which is different from (a) and (b). The compositions can also contain one or more fibers with a melting point or a glass transition temperature above about 130.degree. C. The combination of components in these compositions produce reinforced articles having a particularly good balance of mechanical properties by a very rapid injection mold cycle which is typically less than about 2 minutes from the time the cure of the resin is initiated.
Copending U.S. patent application Ser. No. 278,902, filed June 29, 1981, now abandoned, describes curable molding compositions used for the rapid fabrication of fiber reinforced thermoset resin articles having improved mechanical properties which contain a mixture of (a) a thermosettable organic material containing two or more polymerizable carbon-carbon double bonds which can include a poly(acrylate) characterized by the following empirical formula: ##STR3## wherein R.sub.3 is the hydroxy-free residue of an organic polyhydric alcohol which contains alcoholic hydroxyl groups bonded to different carbon atoms, R.sub.2 and R.sub.4 are independently hydrogen or methyl, and c is 1 to 3, (b) acrylic or methacrylic acid or a functionalized derivative thereof having a molecular weight of less than 300 which is different from (a), and (c) an ethylenically unsaturated monomer which is soluble in and copolymerizable with (a) and (b) and which is different from (a) and (b). The compositions can also contain one or more fibers with a melting point or a glass transition temperature above about 130.degree. C. The combination of components in these compositions produce reinforced articles having a particularly good balance of mechanical properties by a very rapid injection mold cycle which is typically less than about 2 minutes from the time the cure of the resin is initiated.
Copending U.S. patent application Ser. No. 279,450, filed July 1, 1981, describes curable molding compositions used for the rapid fabrication of fiber reinforced thermoset resin articles having improved mold release characteristics which contain (a) a thermosettable organic material containing two or more polymerizable carbon-carbon double bonds which can include a poly(acrylate) characterized by the following empirical formula: ##STR4## wherein R.sub.3 is the hydroxy-free residue of an organic polyhydric alcohol which contains alcoholic hydroxyl groups bonded to different carbon atoms, R.sub.2 and R.sub.4 are independently hydrogen or methyl, and c is 1 to 3, (b) a monoethylenically unsaturated monomer which forms a liquid homogeneous mixture with and is copolymerizable with (a), and (c) a thermosettable organic material containing two or more polymerizable carbon-carbon double bonds having a cross-link density of greater than (a) and containing allyl, vinyl, acrylic and methacrylic types of carbon-carbon double bonds. This application additionally describes an improved process for rapidly fabricating fiber reinforced thermoset resin articles having improved mold release characteristics in molds having shapes that cause problems in releasing the molded article therefrom.
Copending U.S. patent application Ser. No. 279,446, filed July 1, 1981, now abandoned, describes curable molding compositions used for the rapid fabrication of fiber reinforced thermoset resin articles having improved mechanical properties which contain (a) a thermosettable organic material containing two or more polymerizable carbon-carbon double bonds which can include a poly(acrylate) characterized by the following empirical formula: ##STR5## wherein R.sub.3 is the hydroxy-free residue of an organic polyhydric alcohol which contains alcoholic groups bonded to different carbon atoms, R.sub.2 and R.sub.4 are independently hydrogen or methyl, and c is 1 to 3, (b) an ethylenically unsaturated monomer which forms a liquid homogeneous mixture with and is copolymerizable with (a) and is different from (a), and (c) an effective amount of an initiator or a mixture of initiators characterized by a ten-hour half-life temperature, or in the case of a mixture of initiators, an average ten-hour half-life temperature, of greater than about 50.degree. C. and less than 105.degree. C., which produces on decomposition less than 1.0 milliliters of gas per gram of resin as measured at a temperature of 25.degree. C. and a pressure of one atmosphere. This application additionally describes an improved process for rapidly fabricating fiber reinforced thermoset resin articles having improved mechanical properties utilizing the above-described curable molding compositions therein.
While poly(acrylate) resin systems provide for the rapid production of high performance reinforced thermoset resin articles with generally minimal surface requirements such as automotive structural components and the like, it is highly desirable that such reinforced thermoset resin articles have improved surface qualities which will allow the molded article to be used in the rapid production of surface critical articles or parts such as automobile exteriors and the like. It is highly desirable that such surface critical articles or parts have very low profile surfaces which are essentially free of warpage, undulations, fiber prominence or similar defects. There is, therefore, a need to provide curable low profile poly(acrylate) resin systems which exhibit reduced shrinkage during the curing reaction and thereby afford improved surface appearance in the reinforced molded articles obtainable therefrom.
It has been found as a result of the present invention that curable molding compositions containing a poly(acrylate), a polymerizable ethylenically unsaturated monomer which serves to crosslink the poly(acrylate) to a thermoset product, a thermoplastic polymer low profile additive, and a free radical initiator mixture containing at least one initiator with a 10-hour half-life temperature (t.sub.1/2) of greater than about 90.degree. C. and at least one initiator with a 10-hour half-life temperature (t.sub.1/2) of less than about 90.degree. C. exhibit improved shrink control during the curing reaction. Reinforced thermoset resin articles containing one or more fibers with a melting point or a glass transition temperature above about 130.degree. C. and prepared from the curable molding compositions of this invention also exhibit generally improved surface appearance quality.